Time for SNEWS

SN 1987A was the last supernova visible in our skies &#151; and the first from which scientists detected neutrinos.

Hubble Heritage Team (AURA / STScI / NASA)

June 3, 2005Although astronomers estimate a star explodes in our galaxy every few decades, the last Milky Way supernova was observed in 1604. The first sign of the next massive star's collapse will be a blast of neutrinos, not electromagnetic radiation, so astronomers have linked four neutrino-detection facilities located around the world to create an alert network called the SuperNova Early Warning System, or SNEWS, for short.

"SNEWS will be helpful because distant supernovae are routinely observed after they become optically bright enough to be discovered, days after the explosion itself," says Alec Habig of the University of Minnesota, Duluth. Habig presented information about the project Wednesday at the American Astronomical Society meeting in Minneapolis.

The famous supernova SN 1987A in the Large Magellanic Cloud, a satellite of our Milky Way Galaxy, is the first for which neutrinos were detected. The two most sensitive facilities saw interactions from only about 20 of these ghostly particles — but that was enough to confirm astrophysicists' broad ideas of stellar-core collapse and learn more about neutrino properties.

Neutrinos are fundamental particles related to electrons, but they carry no charge and rarely interact with matter. This very feature lets neutrinos escape the incredibly dense center of a collapsing star, but it also makes detecting them quite difficult. Massive facilities are required so neutrinos have a large volume of matter with which to interact, and these detectors must be located deep underground so the sparse neutrino signals aren't swamped by the constant cosmic-ray barrage at Earth's surface.

Four of these experiments — Japan's Super-Kamiokande, the Sudbury Neutrino Observatory in Ontario, Italy's Large Volume Detector, and the Antarctic Muon and Neutrino Detector at the South Pole — send alerts of possible supernova neutrinos to the SNEWS computer at Brookhaven National Laboratory in New York. Multiple detectors would see a real supernova simultaneously, so SNEWS won't generate false alarms, and the scientists believe neutrinos from supernovae in other galaxies will be too weak for these facilities to detect.

A supernova emits about half its neutrinos in a couple of seconds, while the first electromagnetic signal may not come for many hours or even days, depending on the nature of the star's outer layers and obscuring dust between Earth and the star. So, SNEWS offers the possibility of unprecedented early observation of a supernova's birth. A neutrino signal will accompany all core collapses in the Milky Way, but it's possible a supernova could fizzle, collapsing directly to a black hole without visible fireworks. Such an event would also be of great interest to astronomers. Moreover, says Alex Heger of the University of Chicago, neutrinos from any stellar collapse will help constrain models of stellar interiors — in particular, how fast stellar cores rotate.

SNEWS alerts are issued via e-mail, and Habig encourages amateurs to sign up for the service. In the best case, neutrino detectors may locate the event only within a few degrees. This means amateurs with wide-field views may be the first to find the supernova and point more powerful telescopes to the event. Very early data taken by amateurs themselves may be of prime importance, too. Because Galactic supernovae are so rare, it will be especially crucial to see the light's very early turn-on.

"Now," says Habig, "we just need to be patient and wait for a star to explode in our own galaxy, close enough to see in neutrinos." A technical paper on SNEWS has been published in the New Journal of Physics.